chess1

okay, I blew out my 1.5 over the summer. I am turning a 23 mirage plus prop at 5K at about 70 gps. I went to a 1.36 and kept the same prop to see what the RPMs were turning it. Another words was the 23 the max for the boat or could I use more. Well that was it. I have a 502 mpi and with the 1.36 and 23 prop I turned 4200 rpms. My question is do I need a 19 or 21 pitch prop? My math tells me I am going to lose about 5-6 mph either way. Thoughts?

GPM

5000rpm, 1.5 gear and 23 prop @ 70mph is only 4% slip, don't know if that's possible but useing those numbers the 21 pitch with 1.36 gear @ 5k if you can get there should be real close to the same speed.

elise909

Each number you change expect a 200-300 rpm change. A 21 will probally give you 400-500 more rpm. 4200 rpms on your first run a 21 will proballly get you closer to 4600-4700 rpm. Some times you have to take a 19 and have them worked with to get it exactly as you want it. but the 19 will probally give you 800-900 more rpm and put you close to 5000rpm peak hore power on that motor is probally close to that or a little less.
good luck

wtfo

that's the way to do it - if you treat the prop pitch like just another gear it's a simple ratio to figure out the pitch conversion. it is easy to us a prop slip calculator like the one below, plug your current numbers in and, get your theoretical speed, then put your new gear ratio in and click on calculate above pitch. This assumes two things: that the two props are the same efficiency at turning torque into thrust, and that the horsepower and rpm of your engine is the same at wot (for a given set-up max hp pretty much determines top speed). Not perfect, but a good place to start for sure.

I think I remember reading or hearing somewhere that mirage plus props are usually a little over pitched ie act like a little higher than the officially numbered pitch they're given. that puts your calculated slip more like 8%, still probably a bit low. maybe the tack is off a little too... pretty common. ;-)


http://go-fast.com/Prop_Slip_Calculator.htm

ps. where can I get me some of that hore power?

chess1

Thanks for all the replys. I agree that the tac may be slightly off and the 23 mirage plus is more like a 24. The boat came with a 24 4 blade.

jeffswav

Your tach must be off. Your numbers do not match up. Here is another website to check out. http://www.rbbi.com/folders/prop/propcalc.htm

elise909

Sorry for the typo; well said wtfo.

Griff

If you were only turning a 23 M+ with 1.5's, then you should not even be going to a 1.36.

rhsj2006

Yes, I'm curious why youre making the switch to a shorter final ratio. No details given on hull\engine combo, but if you're seeing top speed of 70 at 5k with a 23" prop and neither lagging the engine or bouncing off the rev limiter, your setup was pretty solid.

obnoxus

your going to need to drop to like a 19.

you will have decreased performance all the way around.

much better off repairing the 1:50